1. Field
The present disclosure relates generally to electronics, and more specifically to techniques for generating bias current for switched-capacitor circuits.
2. Background
A switched-capacitor circuit is a circuit that moves charges between different sampling capacitors in order to achieve a desired signal processing function that emulates a resistor network. The switched-capacitor circuit can accurately implement the signal processing function based on ratios of capacitor sizes and a sampling rate for charging and discharging the capacitors, both of which can often be obtained with high precision. Switched-capacitor circuits are widely used to implement various circuit blocks such as analog-to-digital converters (ADCs), digital-to-analog converters (DACs), filters, amplifiers, decimators, and the like.
A switched-capacitor circuit typically includes an active circuit such as an operational amplifier for amplifying an input signal and distributing charge between the sampling capacitors. The active circuit may be biased such that it can adequately respond to the highest expected sampling frequencies under worst-case conditions of semiconductor process, temperature, and voltage (PVT). This biasing causes the active circuit to burn significant power, which may be wasted in lower sampling frequency applications or better PVT conditions.
FIG. 1 illustrates a conventional implementation of a switched capacitor integrator. An input signal 10 feeds a first switch 12, which is coupled to a first capacitor 16 and a second switch 14. The other side of the first capacitor 16 is coupled to a third switch 22 and a fourth switch 24. The other side of the third switch 22 is coupled to an inverting input of an operational amplifier 40. A feedback capacitor 48 is coupled between an output 42 of the operational amplifier 40 and the inverting input. The non-inverting input is coupled to ground. The first switch 12 and fourth switch 24 are controlled by a second phase 60 of a control signal. Similarly, the second switch 12 and third switch 22 are controlled by a first phase 50 of the control signal.
FIG. 2 illustrates the first phase 50 and second phase 60 signals. As can be seen, the first phase 50 is high while the second phase 60 is low and the first phase 50 is low while the second phase 60 is high.
In operation, while the second phase 60 is asserted, the input signal 10 will charge the first capacitor 16 through the first switch 12 while the second switch 14 is open and with the opposite side of the first capacitor 16 coupled to ground through the fourth switch 24. During the time when the first phase 50 is asserted, the second switch 14 and third switch 22 close and the first capacitor 16 will discharge onto the inverting input of the operational amplifier 40 and the second capacitor 48. The combination of the operational amplifier 40 and the second capacitor 48, along with the switched capacitor input function performs an integration function of the input signal 10 such that the output 42 is an integrated signal of the input signal 10.
Operational amplifiers are configured with a response time. For the integrator to perform properly, the response time must be fast enough to respond within the time available (i.e., the asserted time of phase 50) to the operational amplifier. Thus, conventional switched capacitor circuits must be built with an operational amplifier that can respond adequately to the fastest expected sampling frequency (i.e., the smallest asserted time of phase 50). However, if the switched capacitor circuit is running at a slower sampling frequency, the fast operational amplifier will burn more power than is necessary because a slower operational amplifier could have been used.
There is a need for methods and apparatuses for modifying operational amplifiers of switched capacitor circuits such that power of the operational amplifier is adjustable and correlated to a sampling frequency of operation to reduce power of the operational amplifier when it is running slower and decrease response time of the operational amplifier when it is running faster.